1. Field of the Invention
The present invention relates to an X-ray mask and a method of manufacturing the same, and particularly an X-ray mask provided with an alignment mark, which is used for position detection in a process of forming a circuit pattern for transfer, as well as a method of manufacturing the same.
2. Description of the Background Art
In a processes of manufacturing semiconductor devices, a lithography technology utilizing ultraviolet rays has been primarily used for transferring a circuit pattern such as an interconnection pattern onto a semiconductor substrate. For example, the semiconductor devices have been integrated to a higher extent and, for example, DRAMs (Dynamic Random Access Memories) have been improved to have higher densities and higher storage capacities of, e.g., 1 gigabit (Gbit). As a result, it is now required to miniaturize circuit patterns for interconnections or the like to a higher extent.
It is now expected that a lithography technology using X-rays can be a useful technology for transferring such fine circuit patterns. According to the lithography technology using X-rays, X-rays which are used for exposure have a wavelength (soft X-ray: wavelength=5-20 nm) shorter than that of ultraviolet rays which have been used in the prior art, and therefore can transfer fine circuit patterns of a higher resolution than that transferred by the conventional lithography using ultraviolet rays.
The lithography technology using X-rays uses an X-ray mask provided with a circuit pattern for transfer.
FIG. 27 is a cross-sectional view showing a structure of a conventional X-ray mask. Referring to FIG. 27, the conventional X-ray mask will be described below.
In FIG. 27, the conventional X-ray mask includes a substrate 102, a membrane 103, an X-ray absorber 104 and a support ring 101.
Membrane 103 is a substrate allowing passage of X-rays, and is formed on substrate 102. Membrane 103 has a film thickness of 1-3 .mu.m. X-ray absorber 104 is made of a material intercepting transmission of X-rays, and is formed on membrane 103. Substrate 102 is provided with a window 111. A rear surface of membrane 103 is exposed through window 111. In a region located on window 111, X-ray absorber 104 includes a portion 110 for forming a transfer circuit pattern, i.e., a circuit pattern for transfer. Support ring 101 is arranged under substrate 102.
The circuit pattern formed in transfer circuit pattern forming portion 110 of X-ray absorber 104 must have a high position accuracy and a high size accuracy.
The transfer circuit pattern is usually produced through the following steps. First, a resist (not shown) is applied onto X-ray absorber 104. The transfer circuit pattern is written on the resist with an electron beam lithography system. Development is effected on this resist to form a mask pattern of the transfer circuit pattern. Using this mask pattern as a mask, etching is performed to remove X-ray absorber 104, whereby the transfer circuit pattern layer is formed.
In the above step of writing the transfer circuit pattern on the resist with the electron beam lithography system for forming the transfer circuit pattern, the accuracy of position detection significantly affects the accuracies of the position and size of the transfer circuit pattern of the X-ray mask.
The step of writing the transfer circuit pattern with the electron beam includes the following specific steps. First, the X-ray mask including a resist applied over X-ray absorber 104 is attached to a jig called an EB cassette. Then, the X-ray mask is conveyed together with the EB mask into a load-lock chamber (i.e., an area provided for keeping a vacuum around the X-ray mask prior to writing so that the X-ray mask can be conveyed into an area, where writing with the electron beam can be performed, while keeping a vacuum). Then, a vacuum is produced in the load-lock chamber. Subsequently, the X-ray mask is conveyed together with the EB cassette from the load-lock chamber onto a stage in the area for the electron beam writing. The circuit pattern for transfer is written on the resist of the X-ray mask with the electron beam. The writing of the transfer circuit pattern with the electron beam is performed while detecting and referring to positions of alignment marks situated on the EB cassette or the stage.
Usually, there is a difference in temperature between the X-ray mask, the interior of the load-lock chamber and the area for the electron beam writing. When producing a vacuum in the load-lock chamber, the temperatures of the EB cassette and the X-ray mask lower. After the X-ray mask is conveyed onto the stage in the area for the electron beam writing, therefore, the temperatures of the EB cassette and the X-ray mask change until they reach an equilibrium with the temperature of the area where the electron beam writing is performed. As a result, the EB cassette and the X-ray mask conveyed onto the stage expand or shrink due to this change in temperature.
The temperature of the X-ray mask is stabilized to some extent as a certain time elapses. However, a long time is required until the temperature of the EB cassette carrying the X-ray mask or the stage is stabilized. Therefore, it is impossible to write accurately the transfer circuit pattern with the electron beam before the temperature of the entire area for the electron beam writing reaches an equilibrium.
In the conventional process, therefore, the X-ray mask and the EB cassette which were conveyed onto the stage are left standing on the stage for a long time until the entire area for the electron beam writing reaches an equilibrium. The electron beam writing of the transfer circuit pattern is performed after the entire area for the electron beam writing reaches an equilibrium.
The fact that the standing state must be kept until the entire area for the electron beam writing reaches an equilibrium results in increase in manufacturing period of the X-ray mask, and consequently increases a manufacturing cost of the X-ray mask.
In order to reduce the time for which the standing state is kept before writing the transfer circuit pattern with the electron beam, the alignment mark may be formed on the X-ray mask itself instead of the EB cassette or the stage. This is because the alignment mark, which is formed on the X-ray mask, does not change its position once the temperature of only the X-ray mask is stabilized, even before the entire area for the electron beam writing reaches an equilibrium.
Japanese Patent Laying-Open No. 2-166720 (1990) has disclosed an X-ray mask provided with an alignment mark, which is made of a material having a high electron beam reflection coefficient and is formed on the surface of the X-ray mask. FIG. 28 is a perspective view of the X-ray mask proposed in the above publication. Referring to FIG. 28, the X-ray mask already proposed will be described below.
The proposed X-ray mask in FIG. 28 includes a support material 105, a X-ray transmissive material 106 allowing passage of X-rays, an X-ray absorber 104 and alignment marks 107. X-ray transmissive material 106 is formed on support material 105. X-ray absorber 104 is formed on X-ray transmissive material 106. Alignment marks 107 are made of gold films and are formed on X-ray absorber 104.
For forming alignment marks 107 (see FIG. 28), this proposed X-ray mask requires an additional process for forming gold films on X-ray absorber 104. This complicates the process of manufacturing of the X-ray mask, and increases the manufacturing period.
Japanese Patent Laying-Open No. 4-297016 has disclosed an X-ray mask, in which alignment mark providing reference positions are formed on an X-ray absorber. In the X-ray mask disclosed in Japanese Patent Laying-Open No. 4-297016, however, the alignment marks are formed in a region on a membrane where a mask pattern is formed. Therefore, the reference position of the alignment marks shifts due to distortion caused by a heat and a resist stress relief, which are generated or caused when a pattern for forming the mask pattern is written on a resist.